The invention relates to a fuel assembly, in particular for a boiling water nuclear reactor, having a fuel assembly channel which is open at the top and bottom as cladding around a multiplicity of fuel rods which are supported with respect to one another and on the fuel assembly channel by spacers.
For fuel assemblies for boiling water nuclear reactors, it is necessary to be at a sufficiently great distance from what is known as the maximum power for transition to boiling. In this context, the maximum power for transition to boiling is the power at which the formation of steam in the fuel assembly does not yet cause the quantity of heat dissipated to the coolant to be reduced. If the maximum power for transition to boiling is exceeded, a film or even a layer of steam forms on the surface of fuel rods contained in the fuel assembly, and this represents a resistance to heat transfer. Since the quantity of heat generated in the fuel rod is then temporarily no longer completely dissipated, the temperature of the fuel rod rises, until a new thermal equilibrium is established. This may lead to the fuel rod overheating and therefore to thermal overloading of a fuel rod cladding tube. Overheating of this nature must be avoided at all costs, since it would shorten the service life of the fuel rod and therefore of the fuel assembly.
It is known from European Patent EP 0 517 750 B1 to increase the cooling of the fuel rods by segregating water droplets and steam. In this case, the water droplets are diverted, by vanes on the top side of the spacers, out of the center of a sub-channel for a coolant, which is formed by four fuel rods, onto the surface of the fuel rods, while the steam continues to flow upwards in the center of the sub-channel.
However, vanes of this type cannot in fact be disposed in the steam region, (i.e. the upper region of the boiling water fuel assembly, where a considerable part of the cooling water is already present in the form of steam, i.e. the xe2x80x9ctwo-phase regionxe2x80x9d) without having an adverse effect on the hydraulic stability of the flow of coolant. This is because the spacers increase the pressure loss in the two-phase region. In addition to an undesirable reduction in the water throughput as a result of a pressure loss being caused by a fuel assembly with additional internal fittings of this type, the increase in volume which occurs during evaporation prevents the coolant from flowing out. If these negative effects are to be reduced or eliminated altogether, the pressure loss in the two-phase region has to be reduced. Such a reduction can be achieved, inter alia, by enlarging the cross section of flow. The cross section of flow can be enlarged by providing empty positions in the fuel rod grid (see EP 0 517 728 B1). For these reasons, hitherto part-length fuel rods have been used, while vanes of this type have not been deployed. Moreover, the resultant empty positions in the upper fuel assembly region contribute to improving the shutdown performance.
Although shortening some rods reduces the fuel volume in the upper region of the fuel element, it does significantly improve the moderator/fuel ratio and the fuel that is enclosed in the long fuel rods burns more successfully.
It is known that in this case the maximum power for transition to boiling, which is determined by the remaining long rods, decreases. It is stated in Japanese Patent Application JP 1-176986 A (1989) that the enlarged cross section of flow and the correspondingly lower flow resistance leads to a velocity profile of the coolant according to which the film of water flows down the fuel rod surface more slowly and therefore evaporates more quickly, i.e. already contributes to a lower heating power for transition to boiling. Therefore, it is proposed in this document to increase the flow resistance of the spacers which lie above the ends of the part-length rods. This can be achieved by thicker grid webs of the spacers or by protrusions (for example bent-off bottom edges of the grid webs) which project into the space that lies above the shorter fuel rods.
For the same reasons, U.S. Pat. No. 5,229,068 proposes that the upper spacers be constructed from higher webs or for the distances between the spacers to be reduced. Displacement bodies or helically twisted sheet-metal strips above the part-length fuel rods are also intended to completely or partially compensate for the reduction in the flow resistance, so as to restore the original pressure conditions. Above all, it is recommended for the number of spacers in the upper part of the fuel assembly to be increased, i.e. for the distance between the spacers to be reduced continuously or in steps. The greater flow resistance is regarded as a precondition for achieving a higher power for transition to boiling.
However, the hydraulic conditions which are required for sufficient cooling can considerably impair a greater flow resistance and therefore prevent the use of a fuel assembly of this type even if a higher power for transition to boiling were to be possible.
Even other experiments have failed to show a clear relationship between the power for transition to boiling and the flow resistance. Rather, measures are required which enable the remaining long fuel rods, the power of which is high on account of the favorable moderator/fuel ratio, to be effectively cooled without the pressure loss being increased excessively. This is the object of the invention.
It is accordingly an object of the invention to provide a fuel element for a boiling water nuclear reactor which overcomes the above-mentioned disadvantages of the prior art methods of this general type, in which fuel rods are effectively cooled without a pressure loss being increased excessively.
With the foregoing and other objects in view there is provided, in accordance with the invention, a fuel assembly for a boiling water nuclear reactor. The fuel assembly contains a multiplicity of fuel rods having top ends, bottom ends and a fuel assembly channel having a top, a bottom, and openings formed therein at the top and the bottom, The fuel assembly channel encloses the fuel rods. Spacers are connected to the fuel assembly channel and support the fuel rods with respect to one another and on the fuel assembly channel. The spacers are divided into a lower group having identical vertical distances from one another and, an upper group having vertical distances which at least differ from the identical vertical distances of the lower group. A mean distance between the spacers in the upper group is smaller than a mean distance between the spacers in the lower group. Some of the fuel rods are shorter fuel rods being shorter than others of the fuel rods and the shorter fuel rods end below at least two of the spacers. At least some of the spacers of the upper group have an upper side and vanes disposed on the upper side. The vanes are bent obliquely into a flow of a coolant flowing upward between the fuel rods and imparting a turbulence to the flow of the coolant, and none of the vanes projecting into a space which lies above the shorter fuel rods. A lower rod-holding plate is disposed in the fuel assembly channel such that the bottom ends of the fuel rod lie practically at a same height as the lower rod-holding plate.
The invention therefore provides for, in the region of the boiling water fuel assembly which lies above the shorter fuel rods, vanes to be disposed on the top edges of the spacer webs and the number of spacers to be increased (i.e. the distance between the spacers to be reduced) compared to the number of spacers which would result if the spacers were to be disposed at the same distance as in the lower region of the fuel assembly. The vanes are bent obliquely into the flow of coolant flowing along the long fuel rods, in such a manner that they impart turbulence to the flow of coolant, the turbulence guiding the liquid water onto the fuel rods under centrifugal force and keeping the steam away from the surface. This leads to a thicker film of water on the fuel surfaces yet the bent-over vanes do not project so far into the flow of coolant as to protrude into the space above the shorter fuel rods. Therefore, the rapid outgoing flow of steam is virtually unimpeded.
On account of the configuration of the upper spacers and their vanes, the distance between the spacers in the two-phase region becomes a parameter that enables the power for transition to boiling and the pressure loss to be simultaneously optimized.
According to the invention, therefore, the spacers in the lower group are disposed at identical vertical distances from one another, while the spacers in the upper group are disposed at vertical distances which are shorter (at least on average). The lower ends of all the fuel rods lie at one level (for example they may be supported at the bottom on a lower rod-holding plate which is common to all the fuel rods), but some of the fuel rods are shortened and end above spacers above that there are advantageously still at least two spacers. At most the fuel rods which are not shortened are supported on an upper rod-holding plate. In this context, the distance of the bottom and top spacers from the respectively adjacent spacers is not taken into consideration.
Advantageously, the distances between the spacers belonging to the upper group become shorter from the bottom upward; however, it may even be advantageous if the upper group includes two distances that are different from one another and from the constant distance in the lower group.
This is because the fuel rods are set in transverse vibrations by the flow of coolant, and the transverse vibrations are damped by the fuel rods being clamped in the mesh openings of the spacers. If this clamping takes place in planes that are at constant distances from one another, however, pronounced characteristic vibrations are formed with vibration nodes in these planes. However, the characteristic vibrations are detuned and damped if the distances between the spacer planes differ.
In accordance with an added feature of the invention, the spacers include a bottom spacer and a top spacer. A first distance between the bottom spacer and an adjacent next one of the spacers is not taken into consideration, and a second distance between the top spacer and an adjacent next one of the spacers is also not taken into consideration.
The position of the spacers is advantageously determined in such a way that the upper end of each shorter fuel rod is supported virtually directly in a mesh opening of a spacer. In the case of boiling water fuel assemblies, the characteristic vibrations are excited particularly in the lower region, in which only liquid water is flowing, and in this region the friction between the fuel rods and the gridsxe2x80x94with the risk of damage to the fuel rodsxe2x80x94is reduced if the fuel rods are clamped in place at the constant distances of the vibration nodes. In this case, the characteristic vibration is advantageously damped by different distance planes in the upper part of the fuel elements, where the flowing water/steam mixture does not excite the transverse vibrations so strongly.
According to expedient refinements of the invention, only the shortened fuel rods are anchored in the lower rod-holding plate and/or the distances in the upper group of spacers decrease by 10% to 30% from spacer to spacer (based on the distance between the spacers belonging to the lower group). In particular, the shortened fuel rods end just above spacer grids, advantageously, in the case of some fuel rods, in the transition region from the lower group of spacers to the upper group of spacers.
Advantageous configurations of the invention consist in the identical distances between the spacers of the lower group being either 500 to 580 mm, in particular 512 or 568 mm, and the distances in the upper group of spacers decreasing from 500 mm to 359 mm, in particular 410 to 350 mm, 400 to 359 mm, 500 to 390 mm or 450 to 320 mm. It is possible to provide fuel rods with three or four different lengths. Generally, in each case two to five spacers belong to both the lower group and the upper group of spacers.
Further configurations of the invention are achieved by the fact that the spacers contain metal strips which are disposed at right angles to one another, penetrate through one another, form approximately square mesh openings and clamp the fuel rods which have been pushed through the mesh openings resiliently in the horizontal direction. In the upper group, the sheet-metal vanes are positioned next to the penetration locations of the metal strips, on the upper edge of the strips. The vanes imparting in each case turbulent impulses to the coolant flowing vertically through the spacer at adjacent penetration locations, around the penetration locations, (preferably oppositely directed turbulence impulses), and that at each penetration location of the metal strips at least two (advantageously four) sheet-metal vanes are provided.
The vanes are formed in particular by integral parts of the sheet-metal strips (spacer webs). Fuel assemblies according to the invention can also be produced by connecting the side walls of hollow cylindrical sleeves to one another as spacer webs and by the sleeves bearing sheet-metal vanes at their upper ends. The vanes impose a corresponding turbulent impulse on a coolant flowing through between adjacent sleeves.
In accordance with an additional feature of the invention, all of the spacers of the lower group have metal strips disposed at right angles to one another, penetrate through one another, form approximately square mesh openings and clamp the fuel rods which have been pushed through the square mesh openings, resiliently in a horizontal direction.
In accordance with another feature of the invention, the spacers disposed above the shortest fuel rods contain the metal strips disposed at right angles to one another, penetrate through one another, form the approximately square mesh openings and clamp the fuel rods which have been pushed through the mesh openings resiliently in a horizontal direction. The spacers disposed above the shortest fuel rods having the vanes disposed next to penetration locations of the metal strips, on an upper edge of the metal strips.
Fuel assemblies that are configured in accordance with the invention are highly advantageous since they increase the maximum power for transition to boiling without impairing safety and with a relatively low outlay, and also without causing unacceptable nuclear, thermohydraulic or mechanical loads.
In accordance with a further feature of the invention, the spacers are formed of hollow cylindrical sleeves having side walls and they are connected to one another by the side walls. The hollow cylindrical sleeves have upper ends and vanes disposed at the upper ends only at the spacers belonging to the upper group.
The cross section of the fuel assembly channel and of the spacers disposed therein is square in most boiling water fuel assemblies. In this case, a (cylindrical or in particular square) coolant tube is advantageously disposed in the center of the fuel assembly, and this tube also guides liquid cooling water (moderator) in the upper region of the fuel assembly, where it therefore increases the density of the moderator. The fuel rods are in this case disposed in a regular square pattern around the coolant tube, i.e. the spacers form mesh openings which are of the same shape and size and the center points of which in each case form the corners of squares.
Fuel assemblies which have a cross-shaped coolant tube that breaks the channel cross section down into four squares or a configuration containing a plurality of coolant tubes are known. In each case, there is no fuel rod in the center of the fuel assembly channel, but rather the coolant tube configuration extends over the center and the fuel rods are disposed in a it regular square pattern around the configuration.
According to the invention, the center point of the coolant tube configuration may be offset with respect to the center of the channel in the direction of one diagonal of the channel cross section. Alternatively or in addition, the bundle of fuel rods may also be diagonally offset in this way.
In this way, it is possible to compensate for inhomogeneity in the ratio of moderator to fuel, in the burnup and in the power which results from the fuel assemblies often being disposed in the core in such a manner that an instrumentation tube containing measuring probes is adjacent to one corner of the fuel assembly, and a cross-shaped control rod is adjacent to the opposite corner. Often, the widths of the gaps that are formed between the outer sides of adjacent fuel assembly channels and accommodate the control rods and instrumentation tubes are also different. Therefore, the fuel assembly may often be divided into two halves by a diagonal, with fuel rods in one half having a significantly greater tendency toward a transition to boiling than the fuel rods of the other half. It is therefore advantageous for this one half to be cooled more strongly by additionally passing some of the coolant into this one half.
The offset configuration results in corresponding eccentricity in the fuel assembly, which partially compensates such inhomogeneity andxe2x80x94particularly if the length and distribution of the shortened fuel rods is selected appropriatelyxe2x80x94also ensure sufficient cooling and favorable void coefficients.
In accordance with an added feature of the invention, the fuel assembly channel has opposite corners, a center axis, and diagonals defined between pairs of two opposite corners of the fuel assembly channel. The coolant tube has a center axis offset in a direction of one of the diagonals with respect to the center axis of the fuel assembly channel.
In accordance with an additional feature of the invention, the diagonals include a first diagonal and a second diagonal perpendicular to the first diagonal. The center axis of the coolant tube lies on a first side of the second diagonal, and the fuel rods disposed at an edge of the regular square pattern are at a shorter distance from a wall of the fuel assembly channel on the first side of the second diagonal than the fuel rods disposed on a second side of the second diagonal.
In accordance with another feature of the invention, the fuel assembly channel has opposite corners, walls, a center axis, and a diagonal defined between a pair of two opposite corners of the fuel assembly channel. The regular square pattern is offset with respect to the center axis of the fuel assembly channel, in a direction of the diagonal, in such a manner that the fuel rods disposed at an edge of the regular square pattern on a first end of the diagonal are each at a greater distance from one of the walls of the fuel assembly channel than the fuel rods disposed at the edge of the regular square pattern on a second end of the diagonal.
In accordance with a further feature of the invention, there is disposed a larger number of the shorter fuel rods in a first half of the cross section of the fuel assembly channel, which is disposed symmetrically with respect to the second diagonal, than in a second half of the cross section of the fuel assembly channel. And the center axis of the coolant tube lies on that side of the second diagonal, on which the larger number of the shorter fuel rods is situated.
In accordance with another added feature of the invention, a plurality of the shorter fuel rods are disposed directly next to the coolant tube and reside in one half of the cross section of the fuel assembly channel and the cross section of the fuel assembly is symmetrical with respect to the diagonals.
In accordance with a concomitant feature of the invention, at least some of the shortest fuel rods are disposed directly adjacent to the coolant tube.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a fuel element for a boiling water nuclear reactor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.